A wide variety of techniques may be used to analyze biological specimens. Examples of analysis techniques useful in this context include microscopy, microarray analysis (e.g., protein and nucleic acid microarray analysis), and mass spectrometry. Preparing specimens for these and other types of analysis typically includes contacting the specimens with a series of processing liquids. Some of these processing liquids (e.g., staining reagents and counterstaining reagents) may add color and contrast or otherwise change the visual characteristics of invisible or poorly visible specimen components (e.g., at least some types of cells and intracellular structures). Other processing liquids (e.g., deparaffinizing liquids) may be used to achieve other processing objectives. If a specimen is treated with multiple processing liquids, both the application and the subsequent removal of each processing liquid can be important for producing specimens suitable for analysis. In some cases, treating specimens with multiple processing liquids includes manually applying the processing liquids to microscope slides respectively carrying the specimens. This approach to processing specimens tends to be relatively labor intensive and imprecise. “Dip and dunk” automated machines can be used as an alternative to manual specimen processing. These machines automatically process specimens by submerging racks of specimen-bearing slides in open baths of processing liquids. Unfortunately, operation of dip and dunk machines inevitably causes carryover of processing liquids from one bath to another. Over time, this carryover leads to the degradation of the processing liquids. Furthermore, when specimens are immersed in a shared bath, there is a potential for cross-contamination. For example, cells may slough off a specimen on one slide and be transported within a shared bath onto another slide, even on a slide processed much later (e.g., if the cells remain suspended in the bath). This form of contamination can adversely affect the accuracy of certain types of specimen analysis. To mitigate this issue and to address degradation of processing liquids due to carryover, baths of processing liquids in dip and dunk machines typically need to be replaced frequently. Accordingly, these machines tend to consume relatively large volumes of processing liquids, which increases the economic and environmental costs associated with operating these machines. Open baths of processing liquids are also prone to evaporative losses and oxidative degradation of some processing-liquid components. Oxidation of certain components of staining reagents, for example, can alter the staining performance of these components and thereby adversely affect the precision of staining operations.
Some example of conventional histological processing machines that avoid certain disadvantages of dip and dunk machines are known. For example, U.S. Pat. No. 6,387,326 (the '326 patent) to Edwards et al. describes an apparatus for delivering fresh processing liquids directly onto individual slides. The slides are expelled one at a time from a slide storage device onto a conveyor belt. Specimens carried by the slides are individually treated at various stations as the slides move along the conveyor belt. Among other drawbacks, the apparatus described in the '326 patent and similar machines tend to have throughput limitations that make them unsuitable for primary staining applications, such as hematoxylin and eosin (H&E) staining applications. A typical laboratory that performs primary staining, for example, may process hundreds or even thousands of specimens per day. Using the apparatus described in the '326 patent and similar machines for this processing would be unacceptably slow. Furthermore, these machines do not allow for control over staining characteristics. Such control can be important in primary staining applications.